1. The Field of the Invention
The present invention relates to sheave assemblies, and more specifically stuffing box sheave assemblies used on oil and gas wells.
2. Present State of the Art
Wells, such as gas wells, oil wells, and water wells, are created by drilling a deep, narrow hole in the ground and then cementing or otherwise securing a hollow, tubular casing within the hole. The well head is the portion of the casing exposed above the ground surface. A pump or valve is attached to the well head to control the flow of fluid or gas from the well. A stuffing box is typically positioned at the top exposed end of the well head. The stuffing box functions to selectively seal the mouth of the well head. Specifically, the stuffing box operates to produce a seal around objects such as pumping rod or wireline, as discussed below, that are lowered through the mouth of the well head and into the casing.
It is frequently desired to run various types of tooling down the casing. Such tooling can include cameras, vibrators, explosives, various sound generators, and equipment for cleaning the interior of the casing. To facilitate lowering of the tooling within the casing, a wireline is used. The wireline must be able to withstand the highly corrosive environment that is commonly encountered within conventional gas and oil wells. Furthermore, the wireline must be sufficiently strong to withstand the tensile force placed on the wireline when the tooling is lowered hundreds and even thousands of feet within the casing. In addition, the type of wireline used is also dependent upon the type of tooling used. For example, some toolings requires that the wireline carry an electrical current for powering or sending signals back from the tooling.
Due to the above requirements, the wireline can be extremely expensive, even up to several dollars a foot. Most wireline is comprised of stainless steel or other non-corrosive metal. Examples of conventional wireline include coaxial cable, E-line which is an armor cable with one or more conductive lines on the inside, and slick line which is a solid line often made of carbon steel.
During operation, a large continuous spool of wireline is brought to the well site. Although the wireline is relatively flexible, the wireline must be feed into the casing in such a fashion as to avoid kinking the wireline. Kinking can potentially damage or break the wireline. Furthermore, the wireline must facilitate smooth and easy lowering and raising of the tooling within the casing.
To accomplish the above objectives, a lower sheave or pulley is secured near the base of the exposed well head at a position substantially level with the spool of wireline. A stuffing box sheave assembly is attached to the elevated stuffing box. Both of the sheaves include a freely rotatable wheel having a groove formed around the circumference thereof. The groove is configured to receive and retain the wireline. The wireline is drawn around the wheel of the lower sheave and then drawn over the wheel of the stuffing box sheave assembly. The wireline is laid within the groove of the wheels to prevent the wireline from sliding off the wheels. The free end of the wireline is attached to the tooling which is then lowered down into the casing.
Although the sheave assemblies serve a desired function, they have several shortcomings. For example, there are frequent occurrences in which the tooling may accidentally get caught or momentarily stop as it travels down the casing. At these times, slack is produced in the wireline. This slack can cause the wireline to jump out of the groove on the wheel of the stuffing box sheave assembly. Should the tooling then drop, the wireline and sheave assembly can be both badly damaged. On occasion, jumping of the wireline off of the sheave wheel can result in wireline breaking, thereby causing the tooling to freely fall to the bottom of the well. Not only is it extremely expensive to repair broken wireline, but there is extensive down time and expense in fishing the tooling from the bottom of the well. Furthermore, jumping and/or breaking of the wireline creates a hazard to the surrounding workers that are lowering the tooling.
Another problem with conventional sheave assemblies is that the wheel is made out of metal. Since the wheel and wireline are both metal, the engagement therebetween produces a relatively high wear rate on both of the elements. Furthermore, since the metal wheel does not flex or give, the smaller wireline compresses against the weight of the tooling as the wireline passes over the wheel of the stuffing box sheave assembly. Repeated compression and expansion of the wireline results in fatigue and subsequently failure to the wireline.